1. Field of the Invention
The present invention relates to a control system and method for an electric parking brake which is provided on a vehicle such as an automobile and which includes a hill hold function to increase the braking force thereof when the vehicle is stopped on a slope.
2. Related Art
An electric parking brake drives a parking brake for holding a vehicle when it is parked or stopped by using an electric actuator such as a motor.
Since the electric parking brake can be controlled by the driver who controls an electric switch, labor involved in operating the parking brakes can be reduced compared with a hand-operated lever or foot-operated pedal which is generally used.
In addition, in the electric parking brake, the addition of a “hill hold (hill holder)” function has been proposed. This hill hold function is a function to prevent a vehicle from starting to unintentionally move by automatically activating the electric parking brake when the vehicle is stopped on a slope.
Conventionally, as an electric parking brake system, there has been known an electric parking brake system in which when a road surface inclination detected by an inclination sensor (a G sensor) after a vehicle has been stopped on a road surface is larger than a predetermined road surface inclination, a target braking force is calculated based on the road surface inclination, so as to automatically activate electric barking brakes to operate with the target braking force so calculated.
In addition, in the event that a movement of the vehicle is detected by wheel speed sensors after the automatic operation of the brakes, the braking force is automatically increased (the brakes are automatically reapplied). (For example, refer to JP-A-2004-142517)
However, as is described in JP-A-2004-142517, since a predetermined period of time is necessary to accurately calculate a road surface inclination after the vehicle is stopped, it is not possible to prevent the vehicle from starting to move immediately after the vehicle is stopped. To describe this point in greater detail, since there is generated a change in behavior of the vehicle in a pitching direction due to reaction forces of suspensions when the vehicle is stopped, when attempting to calculate a longitudinal acceleration or a road surface inclination at this timing, an error attributed to pitching is generated, whereby an accurate calculation of a road surface inclination becomes impossible. Because of this, it has been an accepted practice to calculate a road surface inclination after a predetermined time period has elapsed since the vehicle was stopped.